Many chemical processes utilize catalytic material to enhance chemical conversion behavior. A catalyst promotes the rate of chemical conversion but does not effect the energy transformations which occur during the reaction. A catalytic chemical reactor therefore must have a facility for energy to flow into or be withdrawn from the chemical process. Often catalytic processes are conducted within tubes which are packed with a suitable catalytic substance. The process gas flows within the tube and contacts the catalytic packing where reaction proceeds. The tube is placed within a hot environment such as a furnace such that the energy for the process can be supplied through the tube wall via conduction. The mechanism for heat transfer with this arrangement is rather tortuous as heat must first be transferred through the outer boundary layer of the tube, conducted through the often heavy gauge wall of the tube and then pass through the inner boundary layer into the process gas. The process gas is raised in temperature and this energy can be utilized by the process for chemical reaction.
The process engineer is often caused to compromise between the pressure drop within the tube reactor with the overall heat transfer and catalytic effectiveness. The inner heat transfer coefficient can be effectively increased by raising the superficial velocity of the process gas. The higher gas velocity therefore improves the thermal effectiveness of the system. However, higher gas velocities increase the system's pressure drop and results in increased compressor sizes and associated operating costs. A reactor must be of sufficient length to allow a reaction to proceed to the required conversion. Utilizing high gas velocities typically results in reactors with large length to width ratios which again results in systems with high pressure drops. The smaller the characteristic dimension of the catalyst particle the higher is the utilization of the catalyst. This is sometimes expressed as a higher effectiveness factor. However, beds formed from small particles exhibit higher pressure drops than similar beds formed from larger particle. So an engineer designs a system with expectable compromises between heat transfer, catalyst utilization, system conversion, and pressure drop. Therefore a reactor for conducting catalytic processes which can promote overall heat transfer and levels of conversion whilst minimizing pressure drop is desired.
The selectivity of some catalytically enhanced chemical reactions is a function of the catalyst particle characteristic length. Such that utilizing large particles, which would reduce pressure drop, results in the formation of an undesirable product. An example of such a reaction is the Fischer Tropsch reaction in which hydrogen reacts with carbon monoxide to yield higher order hydrocarbons. In the Fischer Tropsch reaction the average molecular weight of the product is strongly dependant upon the hydrogen to carbon monoxide which contacts the mixture. Hydrogen has a much higher rate of diffusivity than carbon monoxide such that hydrogen can diffuse into the pores of a catalyst particle more rapidly than carbon monoxide. This can result in a gradient of the ratios of the reactants with the porous matrix. This can result in lighter products and sometimes methane being formed within the particle interior. This phenomenon becomes more prevalent as the catalyst characteristic length becomes larger.
A number of US Patents have been directed to methods of increased heat transfer within reactors and towards enhancing catalyst productivity and selectivity. U.S. Pat. No. 2,512,608 issued to F. J. Buchmann describes a technique in which active catalyst could be preferentially deposited upon the outer layers of alumina particles. The technique utilized the sputtering of iron directly onto the support. The resulting catalyst was claimed to be superior for Fischer Tropsch synthesis in terms of selectivity and catalyst attrition. However issues related to poor metals dispersion and pressure drop are not addresses.
U.S. Pat. No. 4,089,941 issued to B Villemin describes a method to improve the productivity of a nickel based steam reforming catalyst by utilizing structured cylindrical supports. The preferred support arrangement includes an alumina support in the shape of a cylinder containing at least four partitions located in radial planes in which the porosity is in the range 0.08 to 0.2 cm3/g. It is preferred that the partition walls be separated by equal angles. It is claimed that the catalyst topology described results in an inexpensive catalyst which allows higher activity and a lower tendency to deactivate. The reason for the lower rates of deactivation is not given. It is also noted that such a catalyst design offers less resistance to flow and therefor, significantly reduces pressure drop through a packed tube. However, the patent does not teach of methods to adjust catalyst selectivity or methods to control heat transfer and does address the issues relating to the mechanical strength of the catalyst particles.
U.S. Pat. No. 4,599,481 issued to Post, et al. describes a process to produce a catalyst particle in which the active catalytic component is preferentially deposited the outer shell of the catalyst. This type of catalyst has been named egg shell or rim type catalyst. The patent teaches of a technique in which catalyst supports are first placed in a solvent for about 30 minutes. Water is used as an example, such that the solution essentially fills all of the pore volumes within the support. The wetted particle is then placed in a solution which contains a salt of the active catalytic component in solution for a controlled amount of time. The time is sufficient for the dissolved salt to contact the very outermost regions of the catalyst support. The particle is then dried, calcined and reduced as is frequently done in traditional catalyst preparation. The catalyst was used to synthesis higher hydrocarbons from carbon monoxide and hydrogen using the Fischer Tropsch synthesis. It was noted that a rim type catalyst produced appreciably less methane and a higher molecular weight product than a homogeneously impregnated catalyst particle. The technique was further refined and in the subsequent U.S. Pat. No. 4,637,993 again issued to Post et. al. In the second patent it was noted that further reduction of the rim area led to enhanced product distributions and defined a maximum thickness of catalyst impregnation. The technique maximizes productivity and allows large particles to be used which minimize pressure drop in packed columns. However the technique results in reactors in which large volumes of unused catalyst exist and also does not address the issues related to heat transfer.
G.B. Patent No. 2,366,611 issued to Symonds, describes a technique to produce a heat exchanger device. The heat exchanger is formed through the fusion of etched shims to form a structure with two distinct flow channels. The shims can be joined by brazing, welding or diffusion bonding. The patent describes how in certain applications one set of channels may be packed with a suitable catalyst and a reaction performed there. It is stated that this arrangement leads to better productivities than a packed tube in a shell and tube type heat exchanger. The patent does not teach of methods to enhance catalytic activity and selectivity or techniques to minimize reactor pressure drop.
U.S. Pat. No. 5,036,032 issued to Iglesia teaches of the importance of diffusional length scales to the product distribution of the Fischer Tropsch Synthesis. The patent describes a technique to selectivity coat the outermost region of a catalyst with the active salt such that the diffusion length scale can be decoupled from the particle length scale. The technique allows packed beds to be formed from large particles such that pressure drops are minimized. The catalyst support particle is contacted with a liquid molten salt. The molten salt contains the active metal precursor. It is found that depth of penetration can be controlled and predicted through careful choice of contact time. The resulting catalysts are shown to more active and have better selectivities to higher hydrocarbons that a uniformly impregnated catalyst of similar diameter. The patent does not teach of methods to utilise the majority of the catalyst particle or methods to control heat transfer.
U.S. Pat. No. 4,460,704 issued to M. Twigg teaches of a method to produce active catalyst particles which offer low pressure drops when packed in a bed. The technique involves the use of a non porous substrate onto which a catalytic coat is applied. The non porous substrate is in the form of hollow cylinders or hollow cylinders with partition walls in the radial plane. A sol-gel technique is used to apply a thin coat of gamma alumina catalyst coat which is impregnated with the active metal catalyst. The catalyst produced offered good activity with high catalyst utilization. It is thought that the thin catalyst offered a small diffusion resistance allowing a high effectiveness factor. The main advantage of the technique is that the catalyst formed offered significantly higher mechanical strength than a catalyst of similar shape formed through extrusion of the catalyst support only. However, the patent does not teach of methods to adjust catalyst selectivity or methods to control heat transfer.
U.S. Pat. No. 6,211,255 issued to Schanke, describes the suitability of a monolith based reactor system for the Ficher-Tropsch synthesis. The patent teaches that a monolith structure, consisting of essentially a number of straight edged parallel channels, can be coated with a catalytic coat. The thickness of the catalyst coat can be controlled to minimize diffusional resistance and maximize product selectivity. The straight nature of the channels leads to a low tortuosity and hence a low pressure drop. In one embodiment some of the channels are used to allow the passage of a heat transfer fluid to remove the heat of reaction. However the patent does not describe how the catalyst is replaced after deactivation or how the difficulties of manifolding are overcome.
It is therefore a feature of this invention to provide a technique in which catalysts are produce which offer high activity and selectivity whilst minimizing the pressure drop across a bed produced from such particles.
It is therefore a feature of the current invention to provide an improved chemical processor which is suitable for efficiently carrying out chemical reactions.
It is a further feature of this invention to provide a catalytic reactor which offers high rates of heat transfer between the process gas and heat transfer fluid even whilst utilizing low process gas velocities.